1. Field of the Invention
The present invention relates to glass making. In particular, the present invention relates to making molten glass having the viscosity and resistivity needed to achieve a desired level of quality of the glass product and to improve processibility of the melt.
2. Discussion of Background
Glass has been made for thousands of years. It has been used in a wide variety of applications, from windows to art objects, from kitchen utensils to scientific apparatus.
Because of its chemical stability, it is useful as a matrix in which to encapsulate hazardous and radioactive wastes. Borosilicate glass in particular is believed to be an ideal medium for the stable and permanent immobilization of radioactive waste for disposal in geologic repositories.
Typically, glass forming frit and or chemical compounds are fed into an electric melter where electrodes cause the frit to melt by joule heating. As the frit melts and its various ingredients combine, the molten glass can be poured off into molds or canisters. Additional frit is added and melted continuously or periodically. If other materials are to be included in the glass matrix, they are added to the frit simultaneously at predetermined weight ratios.
The two most important properties of the glass melt are the viscosity and resistivity of the combination of constituents. These properties have a fundamental effect on the processibility and quality of the glass product produced from the molten glass.
Viscosity of the melt as a function of temperature is the single most important variable affecting the melt rate of the raw feed, the rate of gas bubble release due to foaming and fining, the rate of homogenization, and the pourability of the glass. If the viscosity is too low, excessive convection currents can occur, increasing corrosion and erosion of the melter materials (refractories and electrodes) and making control of the melter more difficult. For a melter having an operating temperature of about 1150.degree. C., a desirable viscosity range is 20-100 poise. For melters which can reach 1300.degree.-1400.degree. C., viscosities greater than 100 poise are acceptable. Viscosity should range from about 20 poise to not more than about 500 poise, since glasses having viscosities above 500 poise do not pour.
Viscosity is known to be a function of temperature. It is a difficult physical property to measure; and it is impossible to measure viscosity routinely during the production of glass. For example, in the glass industry, the measurement of viscosity is usually performed on a small batch of glass made in a laboratory prior to feeding the glass to a large commercial melter. For glass that incorporates hazardous and radioactive wastes, even premeasurement is impossible.
In electrical melters, the electrical resistivity as a function of temperature is the single most important variable affecting the establishing of the melt itself.
Controlling the process of glass making, including the viscosity and the resistivity of the molten glass, determines the quality of the product. For applications such as encapsulation of radioactive waste, a high quality product is essential. For many other industrial applications, a high quality product is no less desirable.